This invention relates to semi-interpenetrating polymer networks (semi-IPNs) made from a combination of a thermoplastic linear polymer with a short chain thermosetting polymer The invention is more particularly concerned with the provision of a semi-IPN system from the interpenetration of (1) a linear polyimide and (2) an imide oligomer containing unsaturated terminal groups, especially an acetylene-terminated polyimide oligomer, both of which have been initially prepared as the corresponding isoimides. The invention also is directed to a method for achieving such semi-IPN system.
A single polymeric material suitable as a matrix resin for the fabrication of composites that has both toughness and good compressive modulus has not been found. Polymeric matrix resins that have good compressive modulus generally possess high cross-link density which imparts brittleness. The consequent lack of flexibility and toughness renders composites formed of such matrix resins highly susceptible to internal damage due to low velocity impact. On the other hand, tough resins have poor processibility because of their very high melt viscosity at the processing temperature. Because of this high viscosity, tows which are made of very fine fibers (5-7 microns) are poorly impregnated with such resins. Thus, linear aromatic polyimides generally are intractable and have high melt viscosities if they exhibit thermoplasticity at all. One solution is to use blends, namely, blending a resin which has good toughness with one which has good compressive strength However, resin compatibility is a problem.
The method generally used to blend an intractable polyimide with a thermosetting resin is to use the polyamic acid precursor of the polyimide. However, this has the disadvantage that the blend then generates volatiles from the cyclodehydration of the polyamic acid during cure.
Interpenetrating polymer networks (IPNs) are defined as multicomponent resin systems formed by blending of the corresponding monomers of two cross-linking polymers, each of which undergoes cross-linking in the presence of, but independent of, the other
If one of the polymer components is linear and the other is cross-linked, the result is a semi-IPN Selection of the appropriate polymer blends technology is dependent on the specific materials requirements. The general concept of polymer blends encompasses polymer alloys in the Karasz-MacKnight sense, interpenetrating polymer networks (IPNs) in the Sperling-Klempner sense, semi-IPNs and non-interactive diluents developed by Dr. T. L. St. Clair (NASA-Langley), and the reactive diluents approach favored by Dr. F. E. Arnold (AFWAL/MLBP Polymer Branch) The polymer blends approaches in materials development appear to offer a significant advantage.
Among the polymer blends technologies, polymer alloys and semi-IPNs are two related and highly promising approaches to provide toughened, high modulus engineering materials suitable for next generation composite and other structural applications. No adequate solution has been offered by current thermoset and thermoplastic resins due to their respective shortcomings The underlining principle of the polymer alloy and semi-IPN technologies is the enhanced qualities that can emerge by judicious blending of polymer components.
It has been found that there is an enhancement in overall properties by the combination of the linear and cross-linked polymer components. This general approach has been used in U.S. Pat. No. 4,695,610, which discloses semi-2-interpenetrating polymer networks (semi-2-IPNs) prepared by combining a linear polyimidesulfone (PIS) with a cross-linking acetylene-terminated polyimidesulfone (AT-PIS). This combination is stated to result in a semi-2-IPN of improved strength, adhesion and processibility.
However, this approach has certain limitations. The imide moiety in the linear polymer chain generally makes the polymer insoluble and only in very special cases are the linear polyimides soluble enough to be useful for the production of semi-IPNs. Where mixtures of the more soluble polyamic precursor of the linear polyimide is employed in place of the linear polyimide per se, with an acetylene-terminated sulfone, during the processing, volatiles are evolved from the cyclodehydration step of the polyamic acid. This severely limits the laminate fabrication process employing such mixtures because water or alcohol is evolved during the cure. To obtain high quality parts, high pressure is required, as well as special methods to dispose of the condensation by-products. It is accordingly difficult to prepare thick sections of composites expediently because the water must diffuse through the polymer matrix during the cure cycle If high pressures are not employed, the resulting composite would have a high void content.
However, the various polymer blends technologies are expected to produce high performance organic polymer materials for the next generation. As mentioned above, the concept of polymer blending allows the flexibility of utilizing various existing thermoplastic and thermosetting polymers through property tailoring The entire spectrum of polymer blending spans from 100% thermosets (epoxies-like IPNs), to semi-IPNs (with varying ratios of thermoplastic and thermosetting components), and finally to polymer alloying of all thermoplastic polymers. Polymer alloys derived from true thermoplastics (binary or multicomponent) are in theory infinitely reformable.
According to the present invention, isoimide technology is employed for the enhancement of polymer processibility. This was demonstrated with the utilization of difficult-to-process polyimides via their corresponding isoimide forms. Semi-IPN polymer blends developed from these isoimides and other thermoplastic or thermosetting resins possess unusual thermal and thermo-oxidative stabilities. Most importantly, there is no volatile-producing chemical reaction inherent in the processing steps.
One object of the present invention is the provision of a semi-interpenetrating polyimide network (semi-IPN) formed of a combination of a thermoplastic linear polymer with a short chain thermosetting oligomer, resulting in a two-component resin which has better thermomechanical properties than the respective resins made from the individual components.
Another object is the provision of a semi-IPN system from the interpenetration of a linear polyimide and an acetylene-terminated polyimide oligomer, by procedure whereby both of the components are rendered readily processible prior to curing.
Another object is the provision of a semi-IPN system from the interpenetration of a linear polyimide and a maleimide-terminated, nadimide-terminated, or benzocyclobutene-terminated polyimide oligomer, by procedure whereby both of the components are rendered readily processible prior to curing.
Another object is the provision of an improved semi-IPN system of the above type which avoids formation of volatiles and undesired by-products during cure and does not require the imposition of high pressure during curing of composites made from such semi-IPN system.
A still further object of the invention is the provision of matrix resin having good thermomechanical properties in the 450.degree.-600.degree. F. temperature range and is formed from components having better processibility than present state-of-the-art materials.
Yet another object is to provide a two component resin system which upon curing has toughness and good compressive modulus.
A still further object is the provision of procedure for preparation of the above-noted improved semi-interpenetrating polymer systems.